Injecting an RF local oscillator signal into an avalanche photo diode using photons emitted from a light emitting diode

ABSTRACT

The present disclosure generally relates to laser range finders. In one embodiment, a shallow-trench isolation diode operates in a reverse-biased mode. In another embodiment, a poly-defined diode operates in a forward-biased mode. In both embodiments, the diode emits photons in response to a radio frequency current, and the photons are received by an avalanche photo diode during a calibration process.

RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 15/458,969, filed on Mar. 14, 2017, and titled, “UsingIntegrated Silicon LED to Calibrate Phase Offset in Optical Receiver inLaser Range Finder”, which claims priority to U.S. ProvisionalApplication No. 62/307,577, filed on Mar. 14, 2016, and titled, “UsingIntegrated Silicon LED as a Mean to Calibrate Phase Offset in OpticalReceiver in Laser Range Finder”, each of which is incorporated herein byreference.

FIELD

The present disclosure generally relates to laser range finders. In oneembodiment, a shallow-trench isolation diode operating in areverse-biased mode is used for calibration. In another embodiment, apoly-defined diode operating in a forward-biased mode is used forcalibration.

BACKGROUND

Present laser range finders use either the time of flight method forfinding the distance to an object or an RF modulated light signalwherein the phase of the transmitted and reflected light signals arecompared to extract distance using the speed of light. The latter methodis more widespread since it is easier to implement. A factor thatdetermines the distance measurement accuracy is the calibration means.There are many random variables that degrade accuracy such astemperature, component drift, and internal gain settings. To achieve 1mm or better accuracy, frequent calibration is required. This requiresthat the laser range finder be self-calibrating. A requirement forinclusion in a mobile device (e.g., cell phones) is to have the rangefinder be highly integrated and use very little power.

An optoelectronic distance measurement system for use in a mobile device(e.g., mobile phone) includes a lens system, a laser diode emitting ahigh frequency intensity modulated measurement optical light signal, andat least one receiver photodetector for receiving the measurement lightsignal reflected from a measurement object surface. A small portion ofthe optical signal from the laser is coupled to a referencephotodetector and mixed with a reference local oscillator frequencywhich has a small offset frequency from that of the laser modulationfrequency to generate an intermediate frequency reference signal forphase measurement. The receiver photodetector converts the highfrequency optical signal reflected from the measurement object surfaceto a high frequency electrical signal. The receiver high frequencyphotodetector signal is mixed in a high frequency demodulator with thereference local oscillator to generate an intermediate frequency signalwhile preserving the phase delay information. A microcontroller combinedwith a Analog-to-Digital Converter (ADC) measures the phase differenceof the intermediate frequency between the laser transmitting referenceor calibration signal and the received reflected signal. The phasedifference between these signals is directly proportional to thedistance to be measured factoring in the speed of light.

For a laser range finder in mobile phone applications, it is desirablethat the laser emitting power is limited to the Food and DrugAdministration Class 1. It is generally required to have laser opticalpower below 1 milliwatt in the visible spectrum range for continuouswave operation. In order to be able to fit into the form factor of amobile phone, there should not be any mechanical moving part or parts inthe laser range finder. The lens assembly is also limited by thedimensional limitations of a typical mobile phone. It is preferred thatthe diameter of the lens not exceed 4 millimeter, while the height ofentire range finder assembly not exceed 3 millimeter.

Since the optical signal strength at the photodetector drops off at oneover the square of the range distance (1/R²), the photodetector andsubsequent amplifier should have significant dynamic range to ensurethat the range finder has 1 millimeter resolution for a range from 10millimeter to 10 meters. In addition, the surface optical reflectance ofa measurement object is an unknown. A dedicated automatic gain controlmechanism is needed to adjust the gain of the photodetector to ensurethat there is a sufficient signal to noise ratio to recover the phaseinformation.

In addition to recovering the optical signal, it is desirable for therange finder to have the phase information be consistent at differentgain settings. It is also desirable to calibrate the photodetector andsubsequent intermediate frequency filter's phase to ensure that thelaser range finder operation be substantially independent of measurementobject surface reflectance, optical component variations, and electroniccomponent variation and aging effects.

One challenge in building the embodiments of the range finder describedherein include being able to quickly bias the light emitting diode usedin the range finder. Prior art techniques use either off-chip circuitryor on-chip circuitry for this purpose. However, on-chip techniquesinvolve the use of a capacitor stack that consumes a significant amountof space within the chip.

What is needed is a new mechanism for quickly biasing a light emittingdiode to its proper reverse breakdown voltage and at the same timeefficiently introduce a high frequency RF local oscillator signal on tothe DC bias voltage in an integrated circuit, while using less physicalspace on the chip compared to the prior art.

SUMMARY OF THE INVENTION

Embodiments of laser range finders are disclosed. In certainembodiments, a light emitting diode within an integrated circuit in thelaser range finder is biased to its proper reverse breakdown voltage,and at the same time, a high frequency RF local oscillator signal isefficiently introduced on to the bias voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a laser range finder in a mobile device.

FIG. 2 shows an example of a laser range finder system.

FIG. 3 shows an example of an avalanche photo diode with bias andfrequency mixer circuitry.

FIG. 4 is a plot illustrating an example of capacitance of an avalanchephoto diode versus reverse bias.

FIG. 5 shows an example of an equivalent avalanche photo diode circuitmodel.

FIG. 6 are plots illustrating an example of a frequency response of anavalanche photo diode low pass filter.

FIG. 7 shows an example of a bandpass Bode plot for an IF filter.

FIG. 8 is a plot illustrating measurements for a silicon LED accordingto certain aspects of the present disclosure.

FIG. 9 shows an example of a range finder with RF and IF calibrationaccording to certain aspects of the present disclosure.

FIG. 10 shows another example of a range finder with RF and IFcalibration according to certain aspects of the present disclosure.

FIG. 11 shows an example of a range finder with combined RF and IFcalibration according to certain aspects of the present disclosure.

FIG. 12 depicts a prior art bias T circuit.

FIG. 13 depicts a prior art bias circuit within an integrated circuit.

FIG. 14 depicts an embodiment of a bias and RF local oscillatorinjection system for an avalanche photo diode.

FIG. 15 depicts another embodiment of a bias and RF local oscillatorinjection system for an avalanche photo diode.

FIG. 16 depicts a cross-section of a light emitting diode based onshallow trench isolation.

FIG. 17 depicts another cross-section of a light emitting polygatedefined junction diode.

DETAILED DESCRIPTION

The detailed description set forth below, in connection with theappended drawings, is intended as a description of variousconfigurations and is not intended to represent the only configurationsin which the concepts described herein may be practiced. The detaileddescription includes specific details for the purpose of providing athorough understanding of the various concepts. However, it will beapparent to those skilled in the art that these concepts may bepracticed without these specific details. In some instances, well-knownstructures and components are shown in block diagram form in order toavoid obscuring such concepts.

An Improved Laser Range Finder

FIG. 1 shows an example in which a miniaturized laser range finder 102in a cellular phone 101 or other mobile device uses a laser beam 103 tomeasure the distance between the cellular phone 101 and a target object104.

FIG. 2 shows an example of a laser range finder system including a laser201 (e.g., diode laser). The laser 201 is driven at a laser modulationfrequency by a driver 203. The laser modulation frequency 204 isgenerated by a PLL 213 (or another frequency synthesizer) and is between100 MHz to a few GHz, depending on the capability of the laser and theelectronics. The modulation frequency 204 is generated by an offsetfrequency from a local oscillator frequency (labeled LO in FIG. 2) usingthe PLL 213. The offset frequency is generally between 1 kHz˜50 kHz.

The laser range finder system also includes a reference photodetector205, and a target photodetector 206. In some embodiments, each of thephotodetectors may be implemented with an Avalanche Photo Diode (APD)biased by a APD bias voltage (labeled “APD Bias” in FIG. 2) generated bya voltage supply 210.

In operation, a lens 202 focuses the optical light signal (beam) outputby the laser 201 on a target object 220. A small portion of the lightsignal is scattered by the lens 202 and detected by referencephotodetector 205. The reference photodetector 205 converts the detectedlight signal into an electrical signal at the laser modulationfrequency, which is amplified by a TIA 208 and mixed with the localoscillator frequency in a high frequency mixer 211 to generate anintermediate frequency (IF) signal. The IF signal is filtered by an IFfilter 214, amplified by an amplifier 216, and converted into a digitalsignal by an ADC (not shown) for processing in the digital domain by acontroller 218, as discussed further below

The lens 207 focuses the light signal reflected off the target 220 tothe target photodetector 206 for detection by the target photodetector206. The target photodetector 206 converts the detected light signalinto an electrical signal at the laser modulation frequency, which isamplified by a TIA 209 and mixed with the local oscillator frequency ina high frequency mixer 212 to generate an intermediate frequency (IF)signal. The IF signal is filtered by an IF filter 215, amplified by anamplifier 217, and converted into a digital signal by an ADC (not shown)for processing in the digital domain by the controller 218.

The controller 218 computes the phase offset between the referencesignal and the target signal, which provides time of flight informationfor the light signal reflected off of the target. The controller 218then uses the phase offset and speed of light to estimate the distanceto the target. The controller 218 may output the estimated distance toanother processor (e.g., for display to a user).

In order for the laser range finder system to function independent ofthe outside influence in the receiver path, it may be necessary tocalibrate both the RF and IF circuits. In this regard, embodiments ofthe present disclosure use an “on chip” integrated silicon LED tocalibrate the phase offset setting of the photodetector at differentgain settings and the IF filters to ensure resolution of 1 millimeter orbetter. In certain embodiments, an “on chip” silicon LED is used togenerate a reference light signal for calibration, as described furtherbelow. The silicon LED may produce light in the forward mode (IR) or inreverse, avalanche mode (visible light). The use of an “on chip” siliconLED for calibration provides for component cost reduction and sizereduction.

As an example, 300 MHz results in an “electronic wavelength length” of 1meter in free space. In order to resolve better than 1 millimeter, thelaser range finder needs to be able to resolve phase angle between thereceived modulated intensity optical signal and the reference opticalsignal better than 0.36 degree ( 1/1000 of a full circle 360 degree).Measurement errors introduced by the system include temperature,variations in photodetector gain, mismatching and manufacturingvariation of components, and aging effects on the system. Lookup tablesand mathematic algorithms are not capable of ensuring measurementaccuracy under all conditions.

The function of a photodetector in the receiver at RF is to convert thereflected RF modulated optical signal from the measurement objectsurface to an RF electrical signal. A variable gain function is alsoneeded in order to increase the dynamic range requirement of thereceiver. It is also another function for the receiver to be used as ahigh-frequency mixer. An exemplary implementation is shown in FIG. 3.

The exemplary circuit shown in FIG. 3 acts as an optical/electricalconverter, variable gain amplifier, and RF high frequency mixer. In thisexample, the photodetector is implemented with an APD 301 that is biasedby APD bias voltage. When the APD 301 is biased below its junctionbreakdown voltage, it is in linear mode, in which the electrical outputsignal is linearly proportional to the input optical signal. It isdesirable that variable gain is achieved by biasing the APD until auseable signal to noise level is achieved. Typically, the avalanche gainof the APD increases with APD bias. Therefore, adjusting the APD bias isa simple and effective means to achieve gain control. When the biasvoltage is sufficiently low, the linear APD can behave like anattenuator. Such a dynamic attenuator is needed when the measured objectsurface is at very close range to the photodetector. For this case, thesignal needs to be attenuated to avoid overwhelming the subsequentsignal path. This method of gain control is simple yet effective inextending the dynamic range of receiver. Local oscillator (LO) signalscan be also be injected through the use of differential bias-T forperforming mixing.

The exemplary circuit shown in FIG. 3 may be used to implement thephotodetector 205, TIA 208 and mixer 211 shown in FIG. 2. The circuitmay also be used to implement the photodetector 205, TIA 209 and mixer212 in FIG. 2. In this example, the voltage supply 210 may provide theAPD bias, which can be adjusted to achieve variable gain, as discussedabove.

As a consequence of varying the bias on a APD to achieve variable gain,the depletion capacitance of the APD changes. In this regard, FIG. 4shows an example of the capacitance of the APD as a function of areverse bias.

FIG. 5 shows a simple equivalent APD circuit model. The circuit modelconsists of a basic RC low-pass filter with an APD depletion capacitance(Cdepletion) which depends on APD bias and a drift resistor (Rdrift)which has a temperature dependency. The phase response of the APDcircuit is that of a RC low-pass filter as shown in FIG. 6.

A very small change in the APD bias and the corresponding depletioncapacitance and temperature results in significant change in phase angleresponse.

As discussed above, IF is generated by mixing the laser modulationfrequency with a local oscillator frequency. The IF filter (e.g., IF 214or 215) should be a narrow bandpass filter to improve the noise in theIF receiver path. In practice, the bandpass of the IF filter should bearound 1 kHz to 50 kHz. Noise of the IF filter is proportional to itsbandwidth. However, it should not be too narrow to have high attenuationat passband. In this frequency range, the IF filter can be implementedusing electronics with minimal interference from the environment. Forexample, the IF filter can be implemented using an active or passiveanalog filter. The IF filter can also be implemented digitally.

The bandpass IF filter also introduces its own phase error to thesystem. In this regard, FIG. 7 shows an example of the phase response ofa bandpass filter. As can be seen in the example in FIG. 7, the phaseresponse of the filter is particularly sensitive with respect to thefrequency at the center of the passband. Environmental effects,component variations, and aging will significantly alter the phase ofthe resulting signal.

In addition, the center frequency of bandpass filter might have anoffset with respect to the IF frequency due to mismatch of components.This mismatch can skew the phase offset in the IF phase measurement.

In addressing the above challenges, embodiments of the presentdisclosure provide systems and methods to correct the phase offset inthe RF and/or IF signal path of a laser range finder caused by componentmismatch due to, for example, environment (e.g., changes intemperature), process variation during manufacturing, and aging.

Embodiments of the present disclosure use an “on chip” silicon LED togenerate a light signal for calibration. An advantage of an “on chip”silicon LED is that the silicon LED may be integrated with othercomponents of the range finder on the same chip (die) including thephoto detectors, mixers, filters and/or amplifiers. Silicon is anindirect bandgap material. Its light generation efficiency is lowcompared to direct bandgap material, such as gallium arsenide. It isgenerally accepted that silicon LED quantum efficiency is less than10⁻⁵. In this regard, FIG. 8 shows exemplary measurements for a siliconLED, in which the silicon LED is driven with an LED drive current and aphoto detector is used to detect light emitted by the silicon LED. Moreparticularly, FIG. 8 shows the drive current 810 and a ratio 820 of thedetection current of the photo detector over the drive current (which isa measure of coupling efficiency between the silicon LED and photodetector).

In this example, the silicon LED comprises a P/N junction in Silicon OnInsulator (SOI) process with a P/Nwell photo-detector across adielectrically isolated barrier. The silicon LED is fully dielectricallyisolated from the photo detector. In this setup, the coupling efficiencybetween the silicon LED and photo detector is around 10⁻⁷. It is alsoshown that the coupling efficiency is essentially constant over 5decades of LED driving current. LED current can be forward bias orreverse bias. Photo detector current is proportional to the absolutecurrent through silicon LED junction.

In the case of a laser range finder, the optical signal strengthdecreases proportional to 1/R², and proportional to the receiver lensarea intercepting the incoming photons. The received signal strength atthe optical detector (photo detector) can be less than 10⁻⁹ of the laseroptical power. To the optical detector/receiver of laser range finder,the silicon LED light emission/coupling can be efficiently used as ameans to optically/electrically calibrate the receiver path to establish“zero path” offset resulting from environment, mismatch and aging ofcomponents used in the receiver path.

In this regard, FIG. 9 shows an example of a range finder systemincluding a silicon LED 902 for calibration according to certain aspectsof the present disclosure. The range finder system includes a laser 906with its light output modulated at the laser modulation frequency. Therange finder system also includes a target photo detector 901 and areference photo detector 907, each of which may be implemented with anAPD. The reference photo detector 907 establishes the reference pathfrom the laser light. The range finder system also includes a lens 905,which is represented as an optical beam splitter in FIG. 9. The lens 905may pass most of the light signal from the laser 905 to the target 904while scattering a small portion of the light signal to the referencephoto detector 907. The surface scattering will be more than enough tobe picked up by reference photo detector 907 as zero path reference. Theelectrical signal output of the reference photo detector is mixed inmixer 908 with local oscillator signal 909 to generate IF signal 910 ofthe reference path. The light signal from the laser strikes target 904and scattered light is picked up by lens 903 to focus the reflectedsignal onto the target photo detector 901. The electrical signal outputof the target photo detector 901 is mixed in mixer 911 with localoscillator signal 909 to generate the IF signal of the receive path 912.The IF signals in the reference path and receiver path may be filteredby respective IF filters and converted into digital signals byrespective ADCs for processing by a controller. The controller maycompute a phase offset measurement between the reference and receiverpaths based on the received IF signals, as discussed above.

Once the laser measurement is done, the controller may shut down thelaser 906. The silicon LED 902 may then be driven by a driver togenerate a modulated light signal to be picked up (detected)simultaneously by photo detectors 901 and 907. The silicon LED 920 canbe modulated at RF or IF frequency. The electrical signal output of thereference photo detector is mixed in mixer 908 with local oscillatorsignal 909 to generate IF signal 910 of the reference path. Theelectrical signal output of the target photo detector 901 is mixed inmixer 911 with local oscillator signal 909 to generate the IF signal ofthe receive path. The IF signals in the reference path and receiver pathmay be filtered by the respective IF filters and converted into digitalsignals by the respective ADCs for processing by the controller. Thecontroller may compute a calibration phase offset between the referenceand receiver paths based on the received IF signals. The calibrationphase offset may be caused by component mismatch due to, for example,environment (e.g., changes in temperature), process variation duringmanufacturing, and aging. Thus, the controller can subtract out thecalibration phase offset from the phase offset measurement to correctthe phase offset measurement for the component mismatch, resulting in acorrected phase offset, which improves the accuracy of the range finder.The controller may use the resulting corrected phase offset to estimatethe distance from the chip to the target.

The silicon LED 902 is able to calibrate both the photo detector phaseresponse and electronic filter element phase response simultaneously.Light, such as background lighting on target, not modulated at exact RFand IF frequency will not be picked up by the photo detectors due to theselectivity of the receiver.

FIG. 10 shows an example of a range finder system using an silicon LEDin forward bias mode to calibrate at IF. Initially, the laser 1001 isdriven by laser driver 1003 at the laser modulation frequency 1006,which may be offset from the local oscillator frequency 1008 by the IF.The modulation frequency 1006 and local oscillation frequency 1008 aregenerated by PLL 1009. IF frequency is controlled by controller 1010 andit is the difference in frequency between modulation frequency and localoscillator frequency. The light signal (laser beam) output by the laser1001 is focused by lens 1011 onto the target 1012. A small portion oflight is reflected off the lens surface to be picked up by referencephoto detector 1002 to generate reference path signal. The photodetector 1002 may be implemented with an APD that is reverse biased byhigh voltage supply 1005 to adjust the multiplication factor in APD.Controller 1010 may control the high voltage supply 1005 to modulate thegain of APD as the distance and reflectivity of target changes.

The light signal strikes target 1012 and scatters. A portion of thelight signal scatted (reflected) off of the target is picked up by lens1011 to focus onto the target photo detector 1017. Each photo detectorconverts the respective light signal to a respective electrical signal.The electrical signals from the photo detectors 1002 and 1017 passthrough a symmetrical set of TIAs 1007 and 1018, mixers 1013 and 1019,IF filters 1020 and 1022, and amplifiers 1024 and 1026 to controller1010 to be digitized. Controller 1010 computes the phase offset betweenthe reference and received signals to obtain a phase offset measurement.

Once the range measurement is complete, the range finder enters acalibration mode. In the calibration mode, the laser 1001 is shut down.Controller 1010 drives the silicon LED 1014 in forward bias using LEDdriver 1016 at IF frequency. In forward bias mode, the electrical tooptical conversion is slow due to diffusion capacitance of the forwardbias diode junction. In forward bias, the silicon junction voltage isalso low for low power consumption. Silicon LED is fabricated in closeproximity to both photo detector 1002 and 1017 to be optically coupledto both photo detectors 1002 and 1017. Each of the photo detectors 1002and 1017 converts the light received from the silicon LED 1014 into arespective electrical signal. The electrical signals output from thephoto detectors 1002 and 1017 pass through a symmetrical set of TIAs1007 and 1018, mixers 1013 and 1019, IF filters 1020 and 1022, andamplifiers 1024 and 1026 to controller 1010 to be digitized. Controller1010 computes the phase offset between the reference and receivedsignals to obtain a calibration phase offset, and may subtract out thecalibration phase offset from the phase offset measurement to correctfor mismatches in the components at IF. Thus, small mismatches incomponents at IF can be zeroed out in the controller in phase offset.Controller 1010 may estimate the distance from the chip to the targetusing the corrected phase offset.

The sequence of measurement and calibration can be exchangeable,depending on need. Also, the frequency of calibration can be modifieddepending on range measurement accuracy requirement, environmentalconditions, measurement time, and signal to noise ratio (SNR) ofreceived signal at receiver.

FIG. 11 shows an example of a range finder system using a silicon LED tocalibrate both RF and IF. The system setup is similar to that of IFcalibration with exception that the silicon LED 1114 is reverse biased.In the example in FIG. 11, the silicon LED is driven in reverse biasavalanche mode. In avalanche mode, the silicon LED can generate opticalsignals directly at RF frequency. The quantum efficiency is in 10⁻⁵ to10⁻⁶. Light emission is proportional to injected electrical current. Thebandwidth of integrated silicon LED in avalanche mode is limited by thedrift resistance and depletion capacitance of silicon junction. Thereverse breakdown voltage of integrated silicon LED with modern CMOSprocess can range from 6V to 12V depending on CMOS process generation.Reverse breakdown voltage can be generated by a charge pump circuit inLED driver 1116. In this example, the LED driver 1116 may drive thesilicon LED 1114 at the LO frequency or the laser modulation frequency.

In distance measurement mode, the laser 1101 is driven by driver 1103 togenerate modulated light output at modulation frequency 1106. PLL 1109under the control of controller 1110 generates modulation frequency 1106and local oscillator frequency 1108. As discussed above, the modulationfrequency may be offset from the local oscillator frequency by IF.

The light signal output from the laser 1101 is focused onto the target1112 by lens 1111. The lens surface reflects a small portion of lightsignal back to the reference photo detector 1102. A portion of the lightsignal is scattered (reflected) off the target 1112 back towards thereceiver. The lens 1111 focuses the light signal scattered off thetarget to target photo detector 1117. The photo detectors 1102 and 1117may be implemented with APDs 1102 and 1117 that are reversed biased byhigh voltage supply 1105 at high bias voltage 1104. High bias voltage1104 may be adjusted to modulate the multiplication factor of the APDs.In this example, the controller 1110 may control the bias voltage 1104of the high voltage supply 1105 to optimizes APD gain to have a good SNRper target distance and reflectivity. Each photo detector converts therespective light signal to a respective electrical signal. Theelectrical signals from the photo detectors 1102 and 1117 pass through asymmetrical set of TIAs 1107 and 1118, mixers 1013 and 1019, IF filters1120 and 1122, and amplifiers 1124 and 1126 to controller 1110 to bedigitized. Controller 1110 computes the phase offset between thereference and received signals to obtain a phase offset measurement.

In calibration mode, the laser 1101 is switched off. The controller 1110drives the silicon LED 1114 in reverse breakdown using high voltagedriver 1016 at RF frequency. The RF frequency can be either localoscillator frequency 1108 or modulator frequency 1106. The light outputof the silicon LED 1114 is picked up (detected) by the photo detectors1102 and 1117 in close proximity. Each of the photo detectors 1102 and1117 converts the light received from the silicon LED 1114 into arespective electrical signal. The electrical signals from the photodetectors 1102 and 1117 pass through a symmetrical set of TIAs 1107 and1118, mixers 1013 and 1019, IF filters 1120 and 1122, and amplifiers1124 and 1126 to controller 1110 to be digitized. The controller 1110computes the phase offset between the reference and received signals toobtain a calibration phase offset, and subtracts out the calibrationphase offset from the phase offset measurement to correct for mismatchesin the components at RF and IF. Thus, small mismatches in components atRF and IF can be zeroed out in the controller in phase offset. Thecontroller may then compute the distance between the chip and the targetusing the corrected phase offset. Thus, the calibration phase offset(also referred to as zero path phase offset) is applied to correct themeasured phase offset to derive the accurate measurement of distancebetween the chip and target.

In reverse biasing the silicon LED at RF, power consumption by siliconLED will be higher than that of forward biased silicon LED used for IFcalibration. However, using reverse bias at RF, components at both RFand IF can be calibrated at once.

As in IF calibration, the sequence of measurement and calibration can beexchangeable, depending on need. Also, the frequency of calibration canbe modified depending on range measurement accuracy requirement,environmental conditions, measurement time, and signal to noise ratio(SNR) of received signal at receiver.

The controller according to any of the embodiments discussed above maybe implemented using hardwired logic, programmable logic, and/or aprocessor configured to execute code that causes the processor toperform the operations discussed herein. The code may reside in RAMmemory, flash memory, ROM memory, EPROM memory, EEPROM memory,registers, or any other form of storage medium known in the art. Anexemplary storage medium may be coupled to the processor such that theprocessor can read code from storage medium and execute the code toperform the operations discussed herein.

Injecting an RF Local Oscillator Signal into an Avalanche Photo Diode

As described above, the disclosed embodiments of the laser range finderutilize LEDs in the calibration process, such as LED 902 in FIG. 9, LED1014 in FIG. 10, and LED 1114 in FIG. 11, as well as one or more APDs.An APD requires a high voltage reverse bias to achieve the proper gainin sensitivity. Moreover, in order to have the APD act as a mixer, it isalso necessary to introduce a Radio Frequency (RF) Local Oscillator (LO)signal superimposed on top of the high voltage DC bias. For the DC biascomponent, an on-chip charge-pump circuit can be used. Embodiments ofthe invention provide an efficient means to introduce an RF LO signalinto a high voltage biased APD.

FIG. 12 depicts a prior art off-chip circuit for adding an RF signal toa DC voltage in a non-Integrated Circuit (IC) context. Bias-T circuit1200 is used to introduce an RF signal in the presence of a DC voltage.High voltage DC 1211 is introduced through an inductor 1201. Inductor1201 acts as a short circuit to DC bias, while acting as an open circuitto high frequency RF circuit. High frequency RF signal 1212 isintroduced through capacitor 1202. Capacitor 1202 acts as a shortcircuit to high frequency RF signal 1212, while blocking DC current.Thus, DC high voltage is isolated by capacitor 1202. At the output of1213, a high frequency RF signal is superimposed on a DC voltage.

FIG. 13 depicts a prior art circuit for adding an RF signal to a DCvoltage in an IC context (on-chip). An on-chip component voltage ratingmust be observed to ensure long operating life. High voltage 1301 isintroduced from a charge-pump circuit (not shown). The output of APD1312 is connected to amplifier 1302. APD 1312 is isolated from the highvoltage supply and amplifier using resistors 1313 and 1314. APD photocurrent is in the order of 10 pA to 1 uA. A very reasonable resistancevalue of 10 k to 10 Meg can be used as isolation resistors in place ofinductor 1201 in bias-T circuit 1200 of FIG. 12. DC blocking capacitormust be able to tolerate the high voltage without the need for dedicatedprocessing steps while keeping its size manageable.

A capacitor stack 1311 is used to ensure individual capacitor voltagesstay within limit. Additional components, such as junction diodes andresistors, may be needed to further ensure high voltage is dividedevenly between different capacitors and to account for process variationand aging. The net capacitance of the capacitor stack 1311 must be atleast 10 times greater than that of junction capacitance of APD 1312.The high voltage tolerant capacitor array 1311 occupies significant diearea comparing that of APD 1312. It is not unreasonable that in atypical IC process, capacitor stack 1311 array demands a die area thatis 100 to 1000 times greater than that of APD 1312. This large area alsointroduces large parasitic capacitance to die substrate thatsignificantly limits high frequency performance of DC blockingcapacitor. It can be appreciated that the space consumed by capacitorstack 1311 can be substantial.

FIG. 14 shows an embodiment of the invention that overcomes thechallenges of the prior art circuits of FIGS. 12 and 13. LED 1406 (whichcan be LED 902 in FIG. 9, LED 1014 in FIG. 10, and LED 1114 in FIG. 11)is reverse biased. RF signal 1403 is superimposed on to the reversebiased junction diode 1406. In this situation, hot-carrier will occur.Hot-carrier sweeping across the junction barrier under the influence ofbias voltage will result in a weak photo-emission from small junctiondiode 1406. Silicon is not a direct bandgap material in photo-emission.Silicon photo-emission efficiency is very low, from 1e-7 to 1e-4comparing to direct bandgap material. As part of a laser range finder,APD 1407 (which can be APD 301 in FIG. 3, APD 1002 or 1017 in FIG. 10,APD 1102 or APD 1117 in FIG. 11) offers exceptional sensitivity tophoto-emission. Depending on high voltage bias, it offers gain from 1 to100. Here, it is to be understood that APD 1407 can be APD 301, APD1102, or APD 1117 described above in previous embodiments.

It has been known that hot-carrier emission generated in a reverse biasjunction has exceptional frequency response. A high frequency opticalsignal can be efficiently generated depending on junction bias. FIG. 14is an embodiment suitable for use in an IC. It is found thatphoto-emission is directly proportional to the reverse bias current.Reverse bias current is only dependent on the breakdown junctionperimeter edge width, not its junction area. It is found that currentdensity between 5 uA to 500 uA per micron of junction perimeter edgewidth has the best performance in emission efficiency and high frequencyresponse. It is also found that in a typical IC process, couplingefficiency between reverse biased junction emitter to APD can be between1e-4 to 1e-7. Coupling efficiency is defined as APD photo signal currentover the reverse junction photo emitter current. In a laser rangefinder, APD 1407 is biased in breakdown. Photo-emission from reversebiased junction will make APD behave like a switch. RF photo-emissionwill mix with reflected laser light signal received from a distantobject and will result in the desired Intermediate Frequency (IF) forfurther signal processing.

The most obvious advantage in using a simple reverse bias junctiondiode, such as LED 1406, as a photo emitter as a means to inject an RFLO signal into APD 1407 is the die size savings compared to the circuitof FIG. 13. The photon-emitting junction diode only needs to be 5˜50 umin junction perimeter width to achieve the proper coupling of the RFsignal into the APD.

A proper junction spacing between APD and photon-emitting junction diodeis also needed to prevent junction from latchup. In typical siliconprocess, spacing between 5 um to 20 um will be sufficient. As in atypical embodiment, the size of photon-emitting reverse junction diodeis only one one-hundredth of the size of APD. It is also independent ofthe junction capacitance of APD. This enables significant die sizesaving compared to that of a DC-blocking capacitor.

In the embodiment of FIG. 15, an RF LO signal is coupled to APD 1507(which can be APD 301 in FIG. 3, APD 1002 or 1017 in FIG. 10, APD 1102or APD 1117 in FIG. 11) using LED 1506 (which can be LED 902 in FIG. 9,LED 1014 in FIG. 10, and LED 1114 in FIG. 11) biased in a forwarddirection. The main difference compared to the embodiment of FIG. 14 isthat LED 1506 is biased in the forward direction instead of the reversedirection.

FIG. 16 depicts LED 1600, which is an LED embodiment that can be used asLED 1406 in FIG. 14 or 1506 in FIG. 15, or as LED 902 in FIG. 9, LED1014 in FIG. 10, and LED 1114 in FIG. 11. LED 1600 is a shallow-trenchisolation (STI) diode comprising STI areas 1601, N-well 1602,N-diffusion area 1603 (which is the N junction of the diode), andP-diffusion area 1604 (which is the P junction of the diode). Thus, thejunction and N junction are separated by an STI area 1601. AlthoughN-diffusion area 1603 is shown separately from N-well 1602, theyactually are physically the same composition, with N-diffusion area 1603serving as a terminal for an electrical connection outside of LED 1600.

In a forward-bias mode, LED 1600 is relatively slow in turning off. Theperiod of time for the turning off to complete is known as reverserecovery time. The reverse recovery time is needed because the STI area1601 between the P-diffusion area 1604 and the N-diffusion area 1603forces carriers deep within the N-well, below the STI diffusion area1601 that separates the P-diffusion area 1604 and N-diffusion area 1603.In order for the diode to turn off, the injected carriers must firstrecombine. In modern IC processes, the crystalline defect density ofN-well 1602 is very low. The lack of defect recombination center leadsto long recombination time for injected minority carriers. LED 1600 hasthe characteristic that it will emit photos in an amount proportional tothe RF electrical current through the diode junction. However, in aforward-bias mode, LED 1600 is too slow to respond to an RF electricalsignal due to the reverse recovery time needed.

By contrast, when LED 1600 is reverse-biased, LED 1600 can respondalmost immediately to an RF current, without a recovery time. Thejunction capacitance in reverse bias is small, and junction depletionregion is also small. Carriers under high electric field generateshot-carrier current, which emits photons. Hot carrier light emission isusually in visible light spectrum. The resulting photons modulated at RFLO frequency are coupled on chip to the laser range finder APD.

Thus, LED 1600 is well-suited to serve as LED 1406 in FIG. 14 (where LED1406 is reverse-biased), but it is not well-suited to serve as LED 1506in FIG. 15 (where LED 1506 is forward-biased).

FIG. 17 depicts LED 1700, which is an LED embodiment that can be used asLED 1406 in FIG. 14 or 1506 in FIG. 15, or as LED 902 in FIG. 9, LED1014 in FIG. 10, and LED 1114 in FIG. 11. LED 1700 is a poly-defineddiode comprising STI areas 1701, N-well 1702, N-diffusion area 1703,P-diffusion area 1704, and poly silicon gate 1705. \ Unlike in LED 1600,the carriers have a direct path between the P and N junctions, whichsignificantly reduces the volume of silicon injected with carriers torecombine. Also, the conduction path is much closer to the surface ofthe silicon. This effect can be further enhanced by connecting the gateto the corresponding terminal to cause the bulk of the diode to invert.For example, in an N-well diode such as LED 1700, poly silicon gate 1705can be connected to an N-junction terminal.

The use of a forward biased poly-silicon gate defined diode such as LED1700 significantly simplifies the need to generate reverse bias voltageand bias network in the case of hot-carrier emission diode. A simple RFsignal source 1403 or 1503 can be coupled directly to the poly-silicongate defined diode 1700. A dedicated charge pump circuit is notrequired, nor is an RF coupling capacitor bias network required as isthe case in a hot-carrier emission diode such as LED 1600.

A poly-defined diode such as LED 1700 in a forward-bias mode has muchsmaller volume of silicon injected with minority carrier compared to anSTI diode. Only the small region of N-well in between P-diffusion area1704 and N-diffusion area 1703 holds minority carriers. Also, thisregion is substantially close to the surface of silicon, which hasrecombination sites. Therefore, a poly-defined diode such as LED 1700has a much faster reverse recovery time than an STI diode such as LED1600.

On the other hand, a poly-defined diode such as LED 1700 is sensitive toreverse breakdown and are not suitable for emitting photons in responseto an RF signal in reverse-bias mode. Poly-defined diode in forward biaswill have carrier recombination current, which also emits photons. Thisphoton light spectrum is near infrared and determined by the bandgap ofsilicon.

Thus, LED 1700 is well-suited to serve as LED 1506 in FIG. 15 (where LED1506 is forward-biased), but it is not well-suited to serve as LED 1406in FIG. 14 (where LED 1406 is reverse-biased).

Within the present disclosure, the word “exemplary” is used to mean“serving as an example, instance, or illustration.” Any implementationor aspect described herein as “exemplary” is not necessarily to beconstrued as preferred or advantageous over other aspects of thedisclosure. Likewise, the term “aspects” does not require that allaspects of the disclosure include the discussed feature, advantage ormode of operation. The term “coupled” is used herein to refer to thedirect or indirect coupling between two components. It is to beappreciated that the term “lens” as used herein covers a set of lensesconfigured to perform the functions of the “lens”.

The previous description of the disclosure is provided to enable anyperson skilled in the art to make or use the disclosure. Variousmodifications to the disclosure will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other variations without departing from the spirit or scopeof the disclosure. Thus, the disclosure is not intended to be limited tothe examples described herein but is to be accorded the widest scopeconsistent with the principles and novel features disclosed herein.

The invention claimed is:
 1. A laser range finder comprising anintegrated circuit, the integrated circuit comprising: a laser; ashallow trench isolation diode operating in a reverse-biased mode foroutputting a first set of photons in response to an RF local oscillatorsignal; and an avalanche photo detector for generating an electricaloutput in response to the first set of photons and a second set ofphotons generated by the laser and reflected off of a target.
 2. Thelaser range finder of claim 1, wherein a first terminal of the avalanchephoto detector is coupled to a high voltage DC bias signal through afirst resistor.
 3. The laser range finder of claim 2, wherein a secondterminal of the avalanche photo detector is coupled to an amplifierthrough a second resistor.